This invention relates to triangular ring laser gyroscopes and to canceling gyro bias, bias drifts, and noise in such instruments.
In the early 19th century, Fresnel predicted that the speed of light propagating in a moving medium is a function of the velocity of the medium in the direction of propagation. This very small effect was first observed by Fizeau in 1851. Now known as the Fresnel-Fizeau drag effect, the phenomenon is very difficult to measure by interferometric means, but is easily measured using ring laser gyroscopes. Such gyroscopes are sensitive to the apparent frequency shifts produced in the propagating laser beams by the drag effects of gas flow in the laser's gain medium.
Podgorski and Aronowitz ("Langmuir Flow Effects in the Laser Gyro", IEEE J Quantum Electronics, QE-4, 11-18, 1968) have shown that the equilibrium gas flow in a direct current excited helium-neon laser is caused by a wall effect first explained by Langmuir in 1923. Langmuir established that, in a plasma filled gas discharge tube, the equilibrium gas flow profile is the super-position of two gas flow patterns: a flow along the walls from cathode to anode, and a backflow in the center of the discharge tube from anode to cathode.
The Fresnel-Fizeau drag and Langmuir flow effects discussed above degrade the performance of ring laser gyroscopes in two ways. First, the Fresnel-Fizeau drag effect produces an apparent difference, or bias, between the frequencies of laser beams counter-propagating around the gyro through the Langmuir gas flow regions. Second, the Langmuir flow produces a non-uniform gas flow velocity in the discharge region. As temperature changes cause mirror and cavity deformations, the laser beams move about, or wander, within the discharge region, and randomly traverse the non-linear gas flow regions. Fresnel-Fizeau drag effects on the wandering beams therefore produce drifts or variations in the gyro bias and noise.
A frequency bias and a non-uniform gas flow are not necessarily undesirable, as long as the laser beam path remains fixed relative to the gyro block. In fact, means for establishing a known frequency bias are ordinarily incorporated in ring laser gyro designs to avoid "lock in" effects, since a known bias can be compensated for in laser measurements. However, beam wandering in the non-uniform gas flow region of the discharge path is a source of unpredictable and uncompensable variations and drifts in gyro bias and noise which limit the measurement accuracy of the ring laser gyro.
Present-day triangular ring laser gyros compensate for gyro bias by utilizing symmetric split-discharge configurations in which two anodes are symmetrically located in opposite directions from a common cathode along the optical propagation path. Each of the gyro's counter-propagating laser beams successively encounters two regions of the discharge path, one in which the gas flows in the direction of propagation and another in which the gas flows in the opposite direction. As a result, each counter-propagating beam experiences equal but opposite drag effects which essentially cancel the bias effects.
The successful application of the split-discharge technique requires that the two discharge segments be geometrically equivalent and that the discharge currents in the two segments be equal. Maintaining the necessary precise equality in discharge currents is accomplished by sophisticated electronic control circuitry. Additionally, the split-discharge technique requires dual anodes and their associated seals and a large cathode. These required elements increase the manufacturing complexity and cost of the gyro and reduce its reliability.
Moreover, in conventional split-discharge triangular ring laser gyros, the equilibrium flow of lasing medium gas occurs by Langmuir flow within each segment of the split discharge path. Consequently, beam wandering produces undesirable gyro bias drift and noise even where the bias effects may be canceled by the discharge path configuration.
An alternative technique that achieves bias cancellation without the need for current-balancing circuitry and dual anodes, but for a ring laser gyro having a four-segment and non-coplanar optical path, is described in U.S. Pat. No. 4,397,027, to Zampiello et al. The technique includes a single discharge path such that each counter-propagating laser beam traverses one segment of the discharge path in the same direction as the gas flow and another segment in the opposite direction. The drag effects of the gas flow on laser beam frequency are equal and opposite so that the net bias effect is essentially canceled.
However, the approach of Zampiello et al. cannot be practiced in triangular ring laser gyroscopes, in which the optical path has only three segments and the optical path and the discharge path are inherently coplanar. Moreover, such an approach does not address the reduction of bias drift and noise caused by drag effects and beam wandering. The split-discharge approach of conventional triangular ring laser gyroscopes not only does not reduce bias drifts and noise, but also requires the dual-anode and current balancing circuitry discussed above.
Thus, there is a need in the art for a triangular ring laser gyroscope which cancels gyro bias, gyro bias drifts, and noise, and which eliminates the need for multiple anodes, their associated seals, and current balancing electronics. It is therefore an object of this invention to provide a triangular ring laser gyroscope having a self-compensating single discharge path which substantially cancels bias effects, gyro bias drift, and noise.